Stephen was the fourth son of Étienne II Henri, Count of Blois, and Adela, daughter of William the Conqueror. A favorite nephew of King Henry I, he received the county of Mortain (before 1115) and was married to Matilda (1125), the granddaughter of Malcolm III of Scotland and heiress of the county of Boulogne.

When Stephen heard the news about the death of his uncle, he crossed at once to England to seize the crown. In disregard of the rights of King Henry's daughter, Matilda, he was recognized as king in London and Winchester and was crowned on 22 Dec 1135. In 1137 Stephen crossed to Normandy to claim the duchy, but he failed to break the resistance of Geoffroi "Plantagenêt", count of Anjou, who also laid his claim to Normandy, and conceded to truce. In England he was entangled in a war with rebellious barons and offended the Church by arbitrary arrests of several bishops. On 30 Sep 1139 Matilda and her illegitimate half-brother, Robert earl of Gloucester, landed at Arundel and brought most of western England under their control. A battle at Lincoln on 2 Feb 1141 resulted in Stephen's capture by the forces of the Angevins. The church council summoned by papal legate, Henry, bishop of Winchester (Stephen's younger brother), deposed the king by accepting Matilda as "Angliae Normanniaeque domina" (8 Apr 1141).

On 1 Nov 1141 Stephen was released in an exchange for Robert of Gloucester, who had been captured by Stephen's forces, and declared lawful sovereign of England by legatine church council (7 Dec 1141) [2]. The civil war continued until February 1148, when Matilda gave up her struggle and departed for the continent. The state of anarchy engulfed the kingdom with barons becoming increasingly independent from the royal authority. Matilda's son, Henry (later King Henry II) invaded England in 1149 and again in 1153. Stephen fought against Henry and attempted to crown his own son, Eustace, but failed to obtain the consent of Pope Eugenius III. On 6 Nov 1153 Stephen and Henry concluded the treaty of Winchester. Stephen retained the kingship for his lifetime and Henry was acknowledged as heir to Stephen by a charter issued at Westminster on 25 Dec 1153. Biography sources: [3; 4; 5]

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[1]

Both of Stephen's Great Seals read STEPHANUS DEI GRATIA REX ANGLORUM, with the counterseal reading STEPHANUS DEI GRATIA DUX NORMANNORUM. The words "dei gracia (gratia)" ("By the Grace of God") appearing on the Great Seal since the time of William II, were not, as a rule, added to the style of the king in charters and writs until the reign of Henry II.

[2]

A "crown-wearing" ceremony in Canterbury Cathedral was held on 25 Dec 1141 to mark the restoration of Stephen on the throne of England. Many sources count it as a second coronation.